Multipath propagation is a significant source of picture quality degradation in television transmission. The ghosts in the received TV image can be a serious problem with NTSC signals and will be an even more important problem with new modes of television broadcasting and receiving such as Improved Definition TV (IDTV), Extended Definition TV (EDTV), and High Definition TV (HDTV) where these ghosts must be removed to realize the full quality improvement.
Numerous techniques have been studied for the elimination of ghosts in TV images. A good review is contained in a paper by W. Ciciora, et al., entitled "A Tutorial on Ghost Cancellation in Television Systems", IEEE Trans. on Consumer Electronics, Vol. CE-25, February 1979, pp. 9-44. For example, deghosting circuitry is commercially available (in Japan) for EDTV that can achieve from 20 to 30 dB of ghost suppression for ghosts up to 6 dB below the main signal level, as described in the 1989 National Association of Broadcasters Guide to Advanced Television Systems, Washington, D.C., p. 54. However, all previously reported techniques are limited in the magnitude or type of ghosts that can be adequately suppressed with analog signals. In particular, all these techniques fail when there is a null in the channel spectrum. Specifically, with a null, a finite impulse response (FIR) equalizer cannot adequately suppress the ghosts, while an infinite impulse response (IIR) equalizer greatly enhances the noise in the picture. Such a null occurs with a single ghost (reflection) with the same magnitude as the desired signal (e.g., with a nonabsorbing reflector such as most buildings). A null can also occur with multiple ghosts even when they are weaker than the desired signal. Therefore, nulls in the channel spectrum (i.e., zeros in the z-domain that approach the unit circle) are a problem for many TV receivers. Unfortunately, it is difficult for any particular TV user to determine the location of the zeros of the channel from the ghosted picture, so that he or she can obtain an appropriate filter. Thus, not only do current techniques fail to adequately suppress ghosts in many cases, but the user may have to purchase and test the deghosting electronics at home in order to determine if it will work properly.